Occurrence and impact of spore-forming bacteria in the dairy industry

The main aim of this PhD project was to investigate the microbiota of milk and cheese, with a specific focus on the group of spore-forming bacteria. The first project aimed to research highly heat-resistant aerobic spores that can survive the ultra-high temperature process of milk and can be found in UHT milk. For this research project, samples obtained from two industrial UHT milk productions were stored at 20°C and 35°C for 14 months. During this storage period, microbiological analyses were carried out to identify aerobic spore-forming bacteria present in UHT milk. Furthermore, the colour change was assessed as an indicator of organoleptic alteration. This study provided a comprehensive view of the UHT supply chain, identifying species able to form extremely heat-resistant spores. Bacillus licheniformis and Bacillus pumilus are the main species found in both raw milk and UHT milk. These findings may also be relevant in the milk powder industry, where very high process temperatures are used.The second project focused on the possibility of removing lysozyme during the production of Grana Padano PDO cheese. Lysozyme is the only food additive allowed in the cheesemaking process of Grana Padano PDO cheese. Since lysozyme is obtained from hen eggs, it must be declared as an allergen on the label. Consequently, there is significant interest among hard cheese producers in producing Grana Padano PDO cheese without lysozyme. The results showed that it is possible to produce Grana Padano PDO cheese without lysozyme using milk with a low level of anaerobic spore. FFarm management practices, such as silage management and milking procedures, play an important role in reducing spore transmission from the farm environment to raw milk. In this study, a median value below 100 spores/L in raw milk was optimal for producing Grana Padano PDO cheese without lysozyme and avoiding blowing defects.